1. Field of the Invention
The present invention relates to an exhaust system mounted on a motorcycle for purifying exhaust gas emitted from a motorcycle combustion engine and for discharging the purified exhaust gas.
2. Description of the Prior Art
It is known that exhaust gas emitted from the motorcycle combustion engine have hitherto been discharged to the atmosphere after having been purified with a catalytic converter disposed on an exhaust passage. In this known exhaust gas purifying system, the exhaust gas tend to be introduced into the catalytic converter in a condition with its temperature somewhat lowered during the flow through the exhaust passage. Accordingly, immediately after the cold start of the combustion engine, a substantial amount of time is required for the catalyst to be activated to initiate oxidization reaction within the catalytic converter and, therefore, it may often occur that a sufficient purifying function does not take place.
In view of the foregoing, in order for the catalyst to be activated to initiate the oxidization reaction immediately after the cold start of the combustion engine, an exhaust gas purifying system is suggested, in which the catalytic converter is disposed in an upstream portion of the exhaust passage with respect to the direction of flow of the exhaust gas towards the atmosphere. See, for example, the Japanese Laid-open Patent Publication No. 2003-307126.
However, it has been found that if the catalytic converter is disposed in the upstream portion of the exhaust passage, the catalyst converter will have to be disposed inside each of a plurality of exhaust pipes connected to respective engine cylinders, resulting in complication of the structure and increase of the cost. By way of example, in the case of the four-cylinder combustion engine having four upstream exhaust pipes, generally known as header pipes or downpipes, every two upstream exhaust pipes are merged together to provide two intermediate exhaust pipes, which are then merged into a single downstream exhaust pipe. In this merged-pipe system, if the catalytic converter is disposed in the relatively upstream portion of the exhaust passage where the intermediate exhaust pipes are positioned, the total number of the catalytic converters required will be two and those two catalytic converters will be positioned in a lower region of an oil pan at the bottom of the combustion engine.
Considering that in mounting the two catalytic converters in that upstream portion of the exhaust passage, disposition of the two catalytic converters in forward and rearward displaced relation to each other with respect to the longitudinal direction of the exhaust passage will result in unbalanced outputs between the engine cylinders and, therefore, in an attempt to avoid the unbalanced outputs, the two catalytic converters are generally arranged left and right. This arrangement leads to increase of the widthwise space for accommodating those catalytic converters, thus limiting the capacity of the oil pan.
If the two intermediate exhaust pipes are merged into the single downstream pipe at a location, where the same catalytic converters are disposed, and the only catalytic converter is employed rather than the two, problems associated with complication of the structure, high cost and the limitation of the capacity of the oil pan may be eliminated. However, since the two exhaust pipes, into which the four exhaust pipes coming out of the respective engine cylinder are merged, terminate at a location upstream of the catalytic converter and thus have a relatively small length, the engine output characteristic will vary and, in particular, vary at a medium speed region of the combustion engine.